U.S. patent number 10,157,723 [Application Number 15/665,839] was granted by the patent office on 2018-12-18 for multi charged particle beam writing apparatus and method of adjusting the same.
This patent grant is currently assigned to NuFlare Technology, Inc.. The grantee listed for this patent is NuFlare Technology, Inc.. Invention is credited to Tsubasa Nanao, Yukitaka Shimizu.
United States Patent |
10,157,723 |
Nanao , et al. |
December 18, 2018 |
Multi charged particle beam writing apparatus and method of
adjusting the same
Abstract
In one embodiment, a multi charged particle beam writing
apparatus includes an emitter that emits a charged particle beam,
an aperture plate in which a plurality of openings are formed and
that forms multiple beams by allowing the charged particle beam to
pass through the plurality of openings, a blanking plate provided
with a plurality of blankers that each perform blanking deflection
on a corresponding beam included in the multiple beams, a stage on
which a substrate irradiated with the multiple beams, a detector
that detects a reflection charged particle from the substrate,
feature amount calculation circuitry that calculates a feature
amount of an aperture image based on a detection value of the
detector, and aberration correction circuitry that corrects
aberration of the charged particle beam based on the feature
amount.
Inventors: |
Nanao; Tsubasa (Yokohama,
JP), Shimizu; Yukitaka (Yokohama, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
NuFlare Technology, Inc. |
Yokohama-shi |
N/A |
JP |
|
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Assignee: |
NuFlare Technology, Inc.
(Yokohama-shi, JP)
|
Family
ID: |
61070024 |
Appl.
No.: |
15/665,839 |
Filed: |
August 1, 2017 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180040453 A1 |
Feb 8, 2018 |
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Foreign Application Priority Data
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Aug 3, 2016 [JP] |
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2016-152947 |
Dec 6, 2016 [JP] |
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2016-236860 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J
37/153 (20130101); H01J 37/09 (20130101); H01J
37/20 (20130101); H01J 37/304 (20130101); H01J
37/3177 (20130101); H01J 37/045 (20130101); H01J
2237/30472 (20130101); H01J 2237/043 (20130101); H01J
2237/30455 (20130101); H01J 2237/153 (20130101); H01J
2237/1532 (20130101); H01J 2237/24475 (20130101); H01J
2237/1534 (20130101) |
Current International
Class: |
H01J
37/04 (20060101); H01J 37/317 (20060101); H01J
37/304 (20060101); H01J 37/20 (20060101); H01J
37/153 (20060101); H01J 37/09 (20060101) |
Field of
Search: |
;250/305,306,307,309,310,396R,397,492.1,492.2,492.21,492.22,492.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2005-302359 |
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Oct 2005 |
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JP |
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2006-510184 |
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Mar 2006 |
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JP |
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2006-210503 |
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Aug 2006 |
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JP |
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2008-153209 |
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Jul 2008 |
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JP |
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2012-104426 |
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May 2012 |
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JP |
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2014-229481 |
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Dec 2014 |
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JP |
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2015-167212 |
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Sep 2015 |
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JP |
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10-2015-0028415 |
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Mar 2015 |
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KR |
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Other References
Office Action dated Sep. 13, 2018, in Korean Patent Application No.
10-2017-0097549, citing document AO. cited by applicant.
|
Primary Examiner: Souw; Bernard
Attorney, Agent or Firm: Oblon, McClelland, Maier &
Neustadt, L.L.P.
Claims
What is claimed is:
1. A multi charged particle beam writing apparatus comprising: an
emitter that emits a charged particle beam; an aperture plate in
which a plurality of openings are formed and that forms multiple
beams by allowing the charged particle beam to pass through the
plurality of openings; a blanking plate provided with a plurality
of blankers that each perform blanking deflection on a
corresponding beam included in the multiple beams; a limitation
aperture plate that blocks each of beams deflected by the plurality
of blankers so as to enter a beam-OFF state; a stage on which a
substrate is irradiated with the multiple beams; a detector that
detects a reflection charged particle from the substrate; feature
amount calculation circuitry that calculates a feature amount of an
aperture image based on a detection value of the detector; and
aberration correction circuitry that corrects aberration of the
charged particle beam based on the feature amount.
2. The apparatus according to claim 1, wherein the feature amount
calculation circuitry generates an approximate graphic that
approximates the aperture image, and calculates the feature amount
using an area of the aperture image positioned at least one of
inside and outside the approximate graphic, and the aberration
correction circuitry corrects spherical aberration of the charged
particle beam based on the feature amount.
3. The apparatus according to claim 1, wherein the feature amount
calculation circuitry generates an approximate quadrilateral that
approximates the aperture image, and calculates a ratio between a
length of a longer one of diagonal lines of the approximate
quadrilateral and a length of an outer edge of the aperture image
as the feature amount, and the aberration correction circuitry
corrects spherical aberration of the charged particle beam based on
the feature amount.
4. The apparatus according to claim 1, wherein the feature amount
calculation circuitry calculates standard deviation of illuminance
in the aperture image as the feature amount, and the aberration
correction circuitry corrects spherical aberration of the charged
particle beam based on the feature amount.
5. The apparatus according to claim 1, wherein the feature amount
calculation circuitry generates a rectangle that approximates the
aperture image, and calculates a ratio between a long side and a
short side of the rectangle as the feature amount, and the
aberration correction circuitry corrects spherical aberration of
the charged particle beam based on the feature amount.
6. The apparatus according to claim 1, wherein the feature amount
calculation circuitry generates a circle that approximates the
aperture image, and calculates a ratio between a diameter of the
circle and a length of an outer edge of the aperture image as the
feature amount, and the aberration correction circuitry corrects
spherical aberration of the charged particle beam based on the
feature amount.
7. The apparatus according to claim 1, wherein the feature amount
calculation circuitry generates an approximate quadrilateral that
approximates the aperture image, and calculates variation in
lengths of four sides of the approximate quadrilateral as a first
feature amount, and the aberration correction circuitry corrects
astigmatic aberration of multiple beams based on the first feature
amount.
8. The apparatus according to claim 1, wherein the feature amount
calculation circuitry generates an approximate quadrilateral that
approximates the aperture image, and calculates squareness of four
interior angles of the approximate quadrilateral as a second
feature amount, and the aberration correction circuitry corrects
astigmatic aberration of multiple beams based on the second feature
amount.
9. The apparatus according to claim 1, wherein the feature amount
calculation circuitry generates an approximate quadrilateral that
approximates the aperture image, calculates variation in lengths of
four sides of the approximate quadrilateral as a first feature
amount, and calculates squareness of four interior angles of the
approximate quadrilateral as a second feature amount, and the
aberration correction circuitry corrects astigmatic aberration of
multiple beams based on the first feature amount and the second
feature amount.
10. A method of adjusting a multi charged particle beam writing
apparatus, the method comprising: emitting a charged particle beam;
forming multiple beams by allowing the charged particle beam to
pass through a plurality of openings of an aperture plate;
irradiating a substrate placed on a stage with the multiple beams;
detecting a reflection charged particle from the substrate;
calculating a feature amount of an aperture image based on the
detected reflection charged particle; and correcting aberration of
the charged particle beam based on the feature amount.
11. The method according to claim 10, wherein an approximate
graphic that approximates the aperture image is generated, the
feature amount is calculated using an area of the aperture image
positioned at least one of inside and outside the approximate
graphic, and spherical aberration of the charged particle beam is
corrected based on the feature amount.
12. The method according to claim 10, wherein an approximate
quadrilateral that approximates the aperture image is generated,
and a ratio between a length of a longer one of diagonal lines of
the approximate quadrilateral and a length of an outer edge of the
aperture image is calculated as the feature amount, and spherical
aberration of the charged particle beam is corrected based on the
feature amount.
13. The method according to claim 10, wherein standard deviation of
illuminance in the aperture image is calculated as the feature
amount, and spherical aberration of the charged particle beam is
corrected based on the feature amount.
14. The method according to claim 10, wherein a rectangle that
approximates the aperture image is generated, and a ratio between a
long side and a short side of the rectangle is calculated as the
feature amount, and spherical aberration of the charged particle
beam is corrected based on the feature amount.
15. The method according to claim 10, wherein a circle that
approximates the aperture image is generated, and a ratio between a
diameter of the circle and a length of an outer edge of the
aperture image is calculated as the feature amount, and spherical
aberration of the charged particle beam is corrected based on the
feature amount.
16. The method according to claim 10, wherein an approximate
quadrilateral that approximates the aperture image is generated,
and variation in lengths of four sides of the approximate
quadrilateral is calculated as a first feature amount, and
astigmatic aberration of multiple beams is corrected based on the
first feature amount.
17. The method according to claim 10, wherein an approximate
quadrilateral that approximates the aperture image is generated,
and squareness of four interior angles of the approximate
quadrilateral is calculated as a second feature amount, and
astigmatic aberration of multiple beams is corrected based on the
second feature amount.
18. The method according to claim 10, wherein an approximate
quadrilateral that approximates the aperture image is generated,
variation in lengths of four sides of the approximate quadrilateral
is calculated as a first feature amount, and squareness of four
interior angles of the approximate quadrilateral is calculated as a
second feature amount, and astigmatic aberration of multiple beams
is corrected based on the first feature amount and the second
feature amount.
Description
CROSS REFERENCE TO RELATED APPLICATION
This application is based upon and claims benefit of priority from
the Japanese Patent Application No. 2016-152947, filed on Aug. 3,
2016, and the Japanese Patent Application No. 2016-236860, filed on
Dec. 6, 2016, the entire contents of which are incorporated herein
by reference.
FIELD
The present invention relates to a multi charged particle beam
writing apparatus and a method of adjusting a multi charged
particle beam writing apparatus.
BACKGROUND
The increasing integration of LSIs has led to finer and finer
circuit line widths of semiconductor devices. An approach employed
to form desired circuit patterns on semiconductor devices uses a
step-and-repeat exposure system to reduce and transfer, onto a
wafer, a high-precision master pattern (also called a mask, or a
reticle particularly when used in a stepper or scanner) formed on a
piece of quartz. The high-precision original pattern is written
with an electron beam writing apparatus by use of a so-called
electron beam lithography technique.
For example, there are writing apparatuses that use multiple beams.
The use of multiple beams can greatly improve the throughput
because it enables irradiation with more beams at a time (in a
single shot) than in the case where writing is performed using a
single electron beam. In such multi-beam writing apparatuses, for
example, multiple beams are formed by letting an electron beam
emitted from an electron gun pass through an aperture member having
a plurality of apertures. Blanking control is performed on each of
the beams. Unblocked beams are each reduced by an optical system,
and a substrate placed on a movable stage is irradiated with the
resulting beams.
In an aperture member that forms multiple beams, holes in m
columns.times.n rows (where m, n.gtoreq.2) are formed at
predetermined array pitch in a matrix. Thus, the shape (aperture
image) of the overall multiple beams with which a substrate is
irradiated is ideally a rectangular shape. However, under the
influence of spherical aberration of an electron optical system
provided with a writing apparatus, a shape where four sides on the
outer edge of the beam shape project outside or a shape where the
four sides are depressed inside may be caused.
It has been difficult to assess such specific beam shapes and
automatically adjust spherical aberration.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a multi charged particle beam
writing apparatus according to an embodiment of the present
invention.
FIG. 2 is a schematic diagram of an aperture member.
FIGS. 3A to 3C are diagrams that illustrate examples of an aperture
image.
FIGS. 4A to 4C are diagrams that illustrate a method of calculating
a feature amount of an aperture image.
FIGS. 5A and 5B are flow charts for describing a method of
calculating a quadrilateral that approximates an aperture
image.
FIG. 6 is a diagram for describing how to determine an approximate
quadrilateral.
FIG. 7 is a diagram for describing how to determine an approximate
quadrilateral.
FIG. 8 is a diagram for describing how to determine an approximate
quadrilateral.
FIG. 9 is a diagram for describing how to determine an approximate
quadrilateral.
FIGS. 10A and 10B are diagrams for describing how to determine an
approximate quadrilateral.
FIGS. 11A and 11B are diagrams for describing how to determine an
approximate quadrilateral.
FIG. 12 is a graph that illustrates an example of relation between
an aperture image area ratio and aberration correction lens
voltage.
FIGS. 13A to 13C are diagrams that illustrate a method of
calculating a feature amount of an aperture image according to
another embodiment.
FIG. 14 is a schematic diagram that illustrates a multi charged
particle beam writing apparatus according to another
embodiment.
FIG. 15 is a diagram that illustrates an example of change in shape
of an aperture image accompanied by change in astigmatic correction
coil values.
FIG. 16 is a diagram for describing a method of calculating a
feature amount of an aperture image.
FIG. 17 is a diagram that illustrates an example of a calculation
result of a first feature amount.
FIG. 18 is a diagram that illustrates an example of a calculation
result of a second feature amount.
DETAILED DESCRIPTION
In one embodiment, a multi charged particle beam writing apparatus
includes an emitter that emits a charged particle beam, an aperture
plate in which a plurality of openings are formed and that forms
multiple beams by allowing the charged particle beam to pass
through the plurality of openings, a blanking plate provided with a
plurality of blankers that each perform blanking deflection on a
corresponding beam included in the multiple beams, a limitation
aperture plate that blocks each of beams deflected by the plurality
of blankers so as to enter a beam-OFF state, a stage on which a
substrate irradiated with the multiple beams, a detector that
detects a reflection charged particle from the substrate, feature
amount calculation circuitry that calculates a feature amount of an
aperture image based on a detection value of the detector, and
aberration correction circuitry that corrects aberration of the
charged particle beam based on the feature amount.
An embodiment of the present invention will be described below with
reference to the drawings.
FIG. 1 is a schematic diagram of a multi charged particle beam
writing apparatus according to an embodiment of the present
invention. In the present embodiment, a configuration where an
electron beam is used as an example of a charged particle beam is
described. It should be noted that the charged particle beam is not
limited to an electron beam and may be another charged particle
beam, such as an ion beam.
The writing apparatus includes a writing unit W, which irradiates a
substrate 24 as a writing object with an electron beam and writes a
desired pattern, and a control unit C, which controls operation of
the writing unit W.
The writing unit W includes an electron beam barrel 2 and a writing
chamber 20. In the electron beam barrel 2, an electron gun 4, an
illumination lens 6, an aperture member 8 (an aperture plate), a
blanking aperture array 10, a reduction lens 12, an alignment coil
13, a limitation aperture member 14 (a limitation aperture plate),
an objective lens 16, a deflector 18, and an aberration correction
lens 40 are arranged.
The aberration correction lens 40 is arranged between the
illumination lens 6 and the aperture member 8 and for example,
employs a foil lens.
In the writing chamber 20, an XY stage 22 and a detector 26 are
arranged. On the XY stage 22, the substrate 24 as the writing
object is placed. The substrate 24 as the writing object includes
for example, a wafer and a mask for exposure to light, which
transfers a pattern onto the wafer using a
reduction-projection-type photolithographic device or an
extreme-ultraviolet photolithographic device (EUV), such as a
stepper or a scanner where an excimer laser serves as a light
source.
An electron beam 30 emitted from the electron gun 4 illuminates the
whole aperture member 8 approximately vertically through the
illumination lens 6. FIG. 2 is a conceptual diagram that
illustrates a configuration of the aperture member 8. In the
aperture member 8, holes (openings) 80 in m columns (in the y
direction).times.n rows (in the x direction) (where m, n.gtoreq.2)
are formed at predetermined array pitch in a matrix. For example,
the holes 80 in 512 columns.times.512 rows are formed. Each hole 80
is formed to have a rectangular shape identical in size and shape
to the others. Each hole 80 may have a circular shape with an
identical diameter.
The electron beam 30 illuminates a region that includes all of the
holes 80 of the aperture member 8. Part of the electron beam 30
passes through each of the plurality of holes 80 and accordingly,
multiple beams 30a to 30e illustrated in FIG. 1 are formed.
How the holes 80 are arranged is not limited to a case where the
columns and rows are arranged in a lattice configuration as
illustrated in FIG. 2. For example, the holes adjacent to each
other in a longitudinal direction may be alternately arranged in a
staggered configuration.
In the blanking aperture array 10, through-holes are formed to
correspond to the respective arrangement positions of the holes 80
of the aperture member 8 and each through-hole is provided with a
blanker constituted of two electrodes that make a pair. The
electron beams 30a to 30e that pass through the respective
through-holes each undergo deflection according to the voltage
applied by the blanker, independently. Through the deflection, each
beam undergoes blanking control. In this manner, with the blanking
aperture array 10, blanking deflection is performed on each beam of
the multiple beams that have passed through the plurality of holes
80 of the aperture member 8.
The multiple beams 30a to 30e that have passed through the blanking
aperture array 10 are reduced in each individual beam size and
array pitch by the reduction lens 12 and proceed to a central hole
formed in the limitation aperture member 14. The alignment coil 13
adjusts an optical axis so that the multiple beams pass through the
center of the hole of the limitation aperture member 14.
The electron beams deflected by the blankers of the blanking
aperture array 10 are displaced in courses thereof and miss the
position of the hole of the limitation aperture member 14, and are
blocked by the limitation aperture member 14. In contrast, the
electron beams not deflected by the blankers of the blanking
aperture array 10 pass through the hole of the limitation aperture
member 14.
In this manner, the limitation aperture member 14 blocks each of
the beams deflected by the electrodes of the blanking aperture
array 10 so as to enter a beam-OFF state. The beams that have
passed through the limitation aperture member 14 before becoming
OFF after having become ON serve as one shot of beams.
The multiple beams 30a to 30e that have passed through the
limitation aperture member 14 are focused by the objective lens 16
and constitute a pattern image with a desired reduction rate. The
individual beams that have passed through the limitation aperture
member 14 (the overall multiple beams) are collectively deflected
by the deflector 18 in the same direction and the substrate 24 is
irradiated with the deflected beams. The detector 26 detects
reflection electrons (secondary electrons) from the substrate
24.
The multiple beams used in single irradiation are ideally aligned
at the pitch that results from multiplying the array pitch of the
plurality of holes 80 of the aperture member 8 by the
above-described desired reduction rate. The writing apparatus
performs writing operations in a raster scan mode where irradiation
of shot beams is conducted continuously and sequentially, and when
a desired pattern is written, the beams necessary according to the
pattern are controlled through the blanking control so as to be ON.
When the XY stage 22 moves continuously, the irradiation positions
of the beams are controlled by the deflector 18 so as to follow the
move of the XY stage 22.
The control unit C includes a control calculator 50, a storage
device 52, a lens control circuit 54, a control circuit 56, and a
signal acquisition circuit 58. The control calculator 50 acquires
writing data from the storage device 52 and performs a data
conversion process at a plurality of stages on the writing data to
generate shot data unique to the device and outputs the shot data
to the control circuit 56. In the shot data, the irradiation amount
and irradiation position coordinates of each shot and the like are
defined. The control calculator 50 includes a feature amount
calculation unit 51 and an aberration correction unit 53.
The control circuit 56 controls each unit of the writing unit W and
performs a writing process. For example, the control circuit 56
determines irradiation time t by dividing the irradiation amount of
each shot by current density and when a corresponding shot is
performed, applies deflection voltage to the corresponding blankers
of the blanking aperture array 10 so that the beams are ON only
during the irradiation time t.
Further, the control circuit 56 calculates a deflection amount so
that each beam is deflected to the position (coordinates) indicated
by the shot data, and applies the deflection voltage to the
deflector 18. Accordingly, the multiple beams shot at the time
concerned are collectively deflected.
The substrate 24 is irradiated with the multiple beams formed by
passing through the plurality of holes 80 of the aperture member 8.
As described above, in the aperture member 8, the holes 80 in m
columns (in the y direction).times.n rows (in the x direction) (m,
n.gtoreq.2) are formed. Thus, an image indicating the shape of the
overall multiple beams with which the substrate 24 is irradiated,
which may be hereinafter simply referred to as an "aperture image",
is ideally a quadrilateral (a substantially quadrilateral shape)
like multiple beams B1 illustrated in FIG. 3A.
However, in a multi-beam writing apparatus, under the influence of
spherical aberration of a lens, the aperture image may be distorted
and may have a barrel-type shape like beams B2 illustrated in FIG.
3B, where the four sides on the outer edge of the aperture image
project outside, or have a pincushion-type shape like beams B3
illustrated in FIG. 3C, where the four sides on the outer edge are
depressed inside.
In the present embodiment, a feature amount indicating a feature of
an aperture image is calculated and on the basis of the calculated
feature amount, spherical aberration is adjusted. The control
calculator 50 obtains an aperture image on the basis of a detection
value of the detector 26, which is acquired through the signal
acquisition circuit 58. The feature amount calculation unit 51
calculates the feature amount of the aperture image. The aberration
correction unit 53 calculates a voltage applied to the aberration
correction lens 40 on the basis of the calculated feature amount.
The aberration correction unit 53 controls the lens control circuit
54 so that the voltage applied to the aberration correction lens 40
becomes the calculated voltage, corrects spherical aberration, and
performs adjustment so that the aperture image becomes a desired
shape (a quadrilateral).
The feature amount calculation unit 51 generates a quadrilateral
that approximates the obtained aperture image to determine an area
of a region positioned inside the approximate quadrilateral, an
area of a region positioned outside the approximate quadrilateral,
the areas being included in the aperture image, an area of a region
positioned inside the approximate quadrilateral and outside the
aperture image, and the like, and calculates the ratio of these as
a feature amount.
For example, as illustrated in FIG. 4A, as regards the
substantially quadrilateral beams B1, most of the aperture image is
positioned inside an approximate quadrilateral R and the area of a
region 61 positioned outside the approximate quadrilateral R is
small. Further, a region inside the approximate quadrilateral R and
outside the aperture image is zero or almost absent.
As illustrated in FIG. 4B, as regards the barrel-type beams B2,
similar to the substantially quadrilateral beams B1 illustrated in
FIG. 4A, much of the aperture image is positioned inside the
approximate quadrilateral R but the area of a region 62 positioned
outside the approximate quadrilateral R is larger than the area of
the region 61 in FIG. 4A. A region inside the approximate
quadrilateral R and outside the aperture image is zero or almost
absent.
As illustrated in FIG. 4C, as regards the pincushion-type beams B3,
the area of a region 60 inside the approximate quadrilateral R and
outside the aperture image is larger than those of the
substantially quadrilateral beams B1 and the barrel-type beams B2.
Besides, the area of the aperture image positioned outside the
approximate quadrilateral R is exceedingly small.
As described above, as regards the substantially quadrilateral
beams B1 having an ideal beam shape, the area of the region 61
positioned outside the approximate quadrilateral R is small while
the region inside the approximate quadrilateral R and outside the
aperture image is zero or almost absent. Thus, the aberration
correction unit 53 controls the voltage applied to the aberration
correction lens 40 by using the lens control circuit 54 and
corrects the spherical aberration so that the area of the region
positioned outside the approximate quadrilateral R is within a
predetermined range and the region inside the approximate
quadrilateral R and outside the aperture image is smaller than or
equal to a predetermined value.
Next, a method of calculating the quadrilateral R that approximates
an aperture image is described using a flow chart illustrated in
FIGS. 5A and 5B. The approximate quadrilateral R is decided by
calculating four vertices.
When an aperture image is obtained on the basis of the detection
value of the detector 26 acquired through the signal acquisition
circuit 58, on the outline of the aperture image, the feature
amount calculation unit 51 extracts the respective points at which
the X coordinate and the Y coordinate are the maximum and the
respective points at which the X coordinate and the Y coordinate
are the minimum. The point at which the X coordinate is the maximum
is referred to as Xmax, the point at which the X coordinate is the
minimum is referred to as Xmin, the point at which the Y coordinate
is the maximum is referred to as Ymax, and the point at which the Y
coordinate is the minimum is referred to as Ymin. When Xmax, Xmin,
Ymax, and Ymin are each uniquely determined (step S1_Yes), each
amount of spacing among the extracted points is calculated (step
S2).
When all of the calculated amounts of the spacing are larger than
or equal to a predetermined value (step S3_Yes), as illustrated in
FIG. 6, Xmax, Xmin, Ymax, and Ymin can serve as the respective
vertices of a quadrilateral that approximates an aperture image B10
and an approximate quadrilateral is decided (step S4).
When any of the calculated amounts of the spacing is smaller than
the predetermined value (e.g. one-tenth length of one side of an
assumed quadrilateral) (step S3_No), two points with the amount of
the spacing that is smaller than the predetermined value are
regarded as an identical point (step S5). For example, the amount
of the spacing between Xmax and Ymax illustrated in FIG. 7 is
smaller than the predetermined value, and Xmax and Ymax correspond
to an identical vertex of a quadrilateral that approximates an
aperture image B11. When two points are regarded as an identical
point, the number of vertices needed to decide an approximate
quadrilateral (=4) is insufficient and accordingly, the process
proceeds to step S20. The operations after step S20 are described
below.
Depending on an aperture image, Xmax, Xmin, Ymax, and Ymin may not
be each uniquely determined (step S1_No). For example, as for an
aperture image B12 illustrated in FIG. 8, Xmin is not determined
uniquely.
When Xmax or Xmin is undetermined uniquely (step S10_Yes), a point
at which the Y coordinate is the maximum and a point at which the Y
coordinate is the minimum are extracted from among a plurality of
candidates (step S11). For instance, in the example illustrated in
FIG. 8, Xmin is undetermined uniquely and thus, as illustrated in
FIG. 9, the point at which the Y coordinate is the maximum,
Xmin_ymax and the point at which the Y coordinate is the minimum,
Xmin_ymin are extracted from among the candidates that can be
Xmin.
Similarly, when Ymax or Ymin is undetermined uniquely (step
S12_Yes), a point at which the X coordinate is the maximum and a
point at which the X coordinate is the minimum are extracted from
among a plurality of candidates (step S13).
Subsequently, one unselected point is selected from among the
plurality of extraction points on the outline of the aperture image
(step S14). Then, distances from the selected point to the other
extraction points are calculated (step S15). When any of the
calculated distances is shorter than a predetermined value (step
S16_Yes), one of the two points with the distance that is shorter
than the predetermined value is erased (step S17). When there is
any unselected point (step S18_Yes), the process returns to step
S14. Similar operations are performed on all of the extraction
points on the outline of the aperture image. When four extraction
points remain after the operations of steps S14 to S18 (step
S19_Yes), the four points can serve as the vertices of a
quadrilateral that approximates the aperture image and an
approximate quadrilateral is decided (step S4).
For instance, in the example illustrated in FIG. 9, the distance
from Ymax to Xmin_ymax is shorter than the predetermined value and
thus, one of Ymax and Xmin_ymax is erased. Consequently, four
points, which are Xmax, Xmin_ymin, Ymax (or Xmin_ymax), and Ymin,
remain on the outline of the aperture image B12 and an approximate
quadrilateral where the four points serve as its vertices is
decided.
Instead of erasing one of two points with a distance that is
shorter than the predetermined value in step S17, a midpoint
between the two points on the outline may be newly extracted and
the two points may be erased. For instance, in the example in FIG.
9, a midpoint between Ymax and Xmin_ymax on the outline of the
aperture image B12 may be newly extracted and then Ymax and
Xmin_ymax may be erased.
Depending on an aperture image, as a result of the point erasure in
step S17, the number of remaining extraction points becomes three
or less (step S19_No). For example, when the operations of step S14
to S18 are performed on an aperture image B13 illustrated in FIG.
10A, Ymin and Xmin_ymin are processed into one point. Further, Xmax
and Ymax are processed into one point. Accordingly, as illustrated
in FIG. 10B, only three vertices remain, which are points P1, P2,
and P3.
In such a case, a straight line is drawn to connect the remaining
vertices (step S20). For example, as illustrated in FIG. 11A, a
straight line L1 that passes through the points P2 and P3 is drawn,
a straight line L2 that passes through the points P1 and P3 is
drawn, and a straight line L3 that passes through the points P1 and
P2 is drawn.
Subsequently, the shortest distance from a point on the outline of
the aperture image to the straight line drawn in step S20 is
determined. When a plurality of straight lines are drawn, the
shortest of the respective shortest distances to the straight lines
is determined. Then, a point at which the determined shortest
distance is the maximum is regarded as a vertex (step S21). The
operations of steps S20 and S21 are repeated until the number of
vertices reaches four. When the number of vertices reaches four
(step S22_Yes), the four vertices can serve as the vertices of a
quadrilateral that approximates the aperture image and an
approximate quadrilateral is decided (step S4).
For example, a point at which the shortest distance to the straight
lines L1, L2, and L3 illustrated in FIG. 11A is the maximum is a
point P4 illustrated in FIG. 11B. The quadrilateral where the
points P1, P2, P3, and P4 serve as its vertices is a quadrilateral
that approximates the aperture image B13.
In this manner, a quadrilateral that approximates an aperture image
is calculated, and the area of the aperture image positioned
outside the approximate quadrilateral and the area of a region
inside the approximate quadrilateral and outside the aperture image
are determined to control the voltage applied to the aberration
correction lens 40 and correct the spherical aberration.
For example, the relation between the ratio of the area of the
aperture image positioned outside the approximate quadrilateral to
the entire area of the aperture image and the voltage applied to
the aberration correction lens 40 has tendencies illustrated in
FIG. 12. Thus, the aberration correction unit 53 controls the
voltage applied to the aberration correction lens 40 so that the
ratio of the area indicates a desired value by using the lens
control circuit 54. Accordingly, the spherical aberration can be
automatically adjusted so that the aperture image becomes a
quadrilateral (a substantially quadrilateral shape). The ratio of
the area as an adjustment object is extra determined according to
writing accuracy and the like. Further, since the ratio of the area
rises as the applied voltage becomes higher, the ratio of the area
can be adjusted by binary search at the time of the automatic
adjustment.
In this manner, according to the present embodiment, an approximate
quadrilateral of an aperture image (a beam shape) is used to
calculate the feature amount of the aperture image and the
spherical aberration is adjusted on the basis of the feature amount
and thus, the spherical aberration of an optical system in
irradiation with multiple beams can be automatically adjusted with
accuracy.
Although in the above-described embodiment, the area positioned
outside an approximate quadrilateral of an aperture image or the
like serves as the feature amount, the feature amount is not
limited thereto. For example, the ratio between a diagonal line of
the approximate quadrilateral and the length of the outer edge (the
outline) of the aperture image may serve as the feature amount.
FIGS. 13A, 13B, and 13C illustrate diagonal lines DL1 and DL2 of
the approximate quadrilaterals R of the substantially quadrilateral
beams B1, the barrel-type beams B2, and the pincushion-type beams
B3. Compared to the substantially quadrilateral beams B1, as
regards the barrel-type beams B2 and the pincushion-type beams B3,
the ratio of for example, the length of the longer one of the
diagonal lines to the length of the length of the outer edge is
small. The aberration correction unit 53 controls the voltage
applied to the aberration correction lens 40 by using the lens
control circuit 54 and adjusts the spherical aberration so that
this ratio becomes large (larger than or equal to a predetermined
value).
Standard deviation in distribution of brightness (illuminance) in
an aperture image may serve as the feature amount. For example, as
regards substantially quadrilateral beams, the brightness in an
aperture image is approximately uniform and standard deviation is
small. As regards barrel-type beams, it is dark in a central
portion and bright in an outer edge portion, and standard deviation
is large. As regards pincushion-type beams, it is bright in a
central portion and darker toward the outer edge, and standard
deviation is large. The aberration correction unit 53 controls the
voltage applied to the aberration correction lens 40 by using the
lens control circuit 54 and adjusts the spherical aberration so
that the standard deviation becomes small (smaller than or equal to
a predetermined value).
The spherical aberration adjustment method according to the present
embodiment can be applied to cases where an aperture image becomes
shapes other than a square. For example, when the shape of the
aperture image is a rectangle, the shape of the aperture image is
changed on the basis of the ratio between a long side and a short
side and the spherical aberration is adjusted according to the
above-described embodiment. For another example, the spherical
aberration is adjusted so that the standard deviation of the
illuminance becomes small.
When the shape of the aperture image is a circular shape, the
adjustment is performed so that the ratio between the diameter of
the aperture image and the length of the outer edge becomes small.
For another example, the spherical aberration is adjusted so that
the standard deviation of the illuminance becomes small.
Although the embodiment above describes a configuration in which
the spherical aberration is corrected using the aberration
correction lens 40, a foil lens as the aberration correction lens,
and in addition, for example, a combination of a lattice lens and a
correction lens, and an aberration corrector using a multipole
electromagnetic field may also be employed. An aberration
correction lens can decrease spherical aberration components of
beams by causing axially symmetric electric fields and magnetic
fields through an electrostatic lens or a coil.
As illustrated in FIG. 2, in the aperture member 8 that forms
multiple beams, the holes 80 in m columns.times.n rows are formed
at predetermined array pitch in a matrix. The approximate
quadrilateral of an aperture image is ideally a rectangular shape,
and particularly when m=n, is a square. However, under the
influence of astigmatic aberration, the aperture image may be
distorted and the shape of the approximate quadrilateral may fail
to be a square. To increase the viewability of an aperture image, a
focal point may be deviated as suitable only at the time of
adjustment.
Thus, as illustrated in FIG. 14, astigmatic adjustment coils 42 and
44, which correct (adjust) astigmatic aberration of multiple beams,
and a coil control circuit 59, which controls excitation values
(astigmatic correction coil values) to excite the astigmatic
adjustment coils 42 and 44, may be provided to perform the
astigmatic adjustment. The astigmatic adjustment coils 42 and 44
respectively perform the astigmatic adjustment in a first axis
direction and a second axis direction (e.g. the x axis direction
and the y axis direction) that are orthogonal to each other in a
horizontal plane.
As illustrated in FIG. 15, when the astigmatic correction coil
values for the astigmatic adjustment coils 42 and 44 are each
changed, the shape of the approximate quadrilateral of the aperture
image changes.
The feature amount calculation unit 51 calculates the feature
amount that indicates the squareness of the approximate
quadrilateral of the aperture image. The aberration correction unit
53 controls the astigmatic correction coil values set for the
astigmatic adjustment coils 42 and 44 on the basis of the
calculated feature amount by using the coil control circuit 59,
corrects the astigmatic aberration, and performs adjustment so that
the shape of the approximate quadrilateral of the aperture image
becomes a square.
A method of calculating a feature amount indicating the squareness
of an approximate quadrilateral of an aperture image is described.
By the method described in the embodiment above, the feature amount
calculation unit 51 calculates an approximate quadrilateral of an
aperture image on the basis of the detection value of the detector
26. A square is a graphic where the lengths of four sides are equal
and four interior angles are equal (all right-angled). Thus, the
feature amount calculation unit 51 calculates at least one of a
first feature amount, which indicates variation in the lengths of
the four sides of the approximate quadrilateral, and a second
feature amount, which indicates variation in the degrees of the
four interior angles of the approximate quadrilateral.
For example, as regards the approximate quadrilateral illustrated
in FIG. 16, the feature amount calculation unit 51 calculates
standard deviation (or variance) in lengths H1, H2, H3, and H4 of
the four sides as the first feature amount. Changing the setting of
the astigmatic correction coil values and calculating the first
feature amount are repeated so that the first feature amount
becomes small. FIG. 17 is a graph that illustrates the relation
between the astigmatic correction coil values set for the
astigmatic adjustment coils 42 and 44 and the first feature amount
calculated. By causing the astigmatic correction coil values set
for the astigmatic adjustment coils 42 and 44 to be the values
indicated by the broken line in FIG. 17, the lengths H1, H2, H3,
and H4 of the four sides of the approximate quadrilateral is caused
to be approximately equal.
Further, for example, as regards the approximate quadrilateral
illustrated in FIG. 16, the feature amount calculation unit 51
calculates the sum of the inner-product squares of four interior
angles .theta..sub.1, .theta..sub.2, .theta..sub.3, and
.theta..sub.4 as the second feature amount. The sum of the
inner-product squares is calculated using the following expression:
(cos .theta..sub.1).sup.2+(cos .theta..sub.2).sup.2+(cos
.theta..sub.3).sup.2+(cos .theta..sub.4).sup.2
The second feature amount indicates the squareness of the four
interior angles .theta..sub.1, .theta..sub.2, .theta..sub.3, and
.theta..sub.4 and when the approximate quadrilateral is a square,
the second feature amount is zero. Changing the setting of the
astigmatic correction coil values and calculating the second
feature amount are repeated so that the second feature amount
becomes small. FIG. 18 is a graph that illustrates the relation
between the astigmatic correction coil values set for the
astigmatic adjustment coils 42 and 44 and the second feature amount
calculated. By causing the astigmatic correction coil values set
for the astigmatic adjustment coils 42 and 44 to be the values
indicated by the broken line in FIG. 18, the four interior angles
.theta..sub.1, .theta..sub.2, .theta..sub.3, and .theta..sub.4
become approximately 90.degree..
Both of the first feature amount and the second feature amount may
be used to control the astigmatic correction coil values and
perform the astigmatic adjustment. In this case, the second feature
amount can be adjusted so as to be small by changing the first axis
and the first feature amount can be adjusted so as to be small by
changing the second axis.
When the shape of the approximate quadrilateral of an ideal
aperture image is a rectangle, the astigmatic correction coil
values are controlled so that the above-described first feature
amount indicates a predetermined value based on the ratio between a
long side and a short side. When the shape of the approximate
quadrilateral is a shape other than a rectangular shape, and when
it is for example, a circular shape, the astigmatic correction coil
values may be adjusted on the basis of the ratio between the
diameter of the aperture image and the length of the outer edge,
and when it is a hexagon, the adjustment may be performed by
regarding the hexagon as a circular shape. Further, adjustment may
be performed on the optical system so as to decrease the standard
deviation (variance) in illuminance of the aperture image.
Instead of the aperture member 8 or the blanking aperture array 10,
a plate for adjustment provided with an opening may be placed to
perform the astigmatic adjustment. In this case, a feature amount
appropriate to the opening provided in the plate for adjustment is
chosen.
At least part of the control calculator 50 including the feature
amount calculation unit 51 and the aberration correction unit 53
described in the above embodiments may be implemented in either
hardware including electric circuits (circuitry) or software. When
implemented in software, a program that realizes at least part of
functions of the control calculator 50 may be stored on a recording
medium such as a flexible disk or CD-ROM and read and executed by a
computer including electric circuits (circuitry). The recording
medium is not limited to a removable recording medium such as a
magnetic disk or optical disk, but may be a non-removable recording
medium such as a hard disk device or memory.
While certain embodiments have been described, these embodiments
have been presented by way of example only, and are not intended to
limit the scope of the inventions. Indeed, the novel methods and
systems described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in
the form of the methods and systems described herein may be made
without departing from the spirit of the inventions. The
accompanying claims and their equivalents are intended to cover
such forms or modifications as would fall within the scope and
spirit of the inventions.
* * * * *